docs: def get_bounding_box(self):
          """
          Compute bounding box for each surface element, and then return
          the bounding box of all surface elements' bounding boxes.
  
          :return:
              A tuple of four items. These items represent western, eastern,
              northern and southern borders of the bounding box respectively.
              Values are floats in decimal degrees.
          """
          lons = []
          lats = []
          for surf in self.surfaces:
              west, east, north, south = surf.get_bounding_box()
              lons.extend([west, east])
              lats.extend([north, south])
          return utils.get_spherical_bounding_box(lons, lats)